Good, reliable communications among personnel engaged in hazardous environmental activities, such as fire fighting, are essential for accomplishing their missions while maintaining their own health and safety. Working conditions may require the use of a pressurized air delivery system such as, for instance, a Self Contained Breathing Apparatus (SCBA) mask and air delivery system, a Self Contained Underwater Breathing Apparatus (SCUBA) mask and air delivery system, or an aircraft oxygen mask system. However, even while personnel are using such pressurized air delivery systems, it is desirable that good, reliable communications be maintained and personnel health and safety be effectively monitored.
FIG. 1 illustrates a simple block diagram of a prior art system 100 that includes a pressurized air delivery system 110 coupled to a communication system 130. The pressurized air delivery system typically includes: a breathing mask 112, such as a SCBA mask; an air cylinder (not shown); a regulator 118; and a high pressure hose 120 connecting the regulator 118 to the air cylinder. Depending upon the type of air delivery system 110 being used, the system 110 may provide protection to a user by, for example: providing the user with clean breathing air; keeping harmful toxins from reaching the user's lungs; protecting the user's lungs from being burned by superheated air inside of a burning structure; protecting the user's lungs from water; and providing protection to the user from facial and respiratory burns. Moreover, in general the mask is considered a pressure demand breathing system because air is typically only supplied when the mask wearer inhales.
Communication system 130 typically includes a conventional microphone 132 that is designed to record the speech of the mask wearer and that may be mounted inside the mask, outside and attached to the mask, or held in the hand over a voicemitter port on the mask 112. Communication system 130 further includes a communication unit 134 such as a two-way radio that the mask wearer can use to communicate her speech, for example, to other communication units. The mask microphone device 132 may be connected directly to the radio 134 or through an intermediary electronic processing device 138. This connection may be through a conventional wire cable (e.g., 136), or could be done wirelessly using a conventional RF, infrared, or ultrasonic short-range transmitter/receiver system. The intermediary electronic processing device 138 may be implemented, for instance, as a digital signal processor and may contain interface electronics, audio amplifiers, and battery power for the device and for the mask microphone.
There are some shortcomings associated with the use of systems such as system 100. These limitations will be described, for ease of illustration, by reference to the block diagram of FIG. 2, which illustrates the mask-to-radio audio path of system 100 illustrated in FIG. 1. Speech input 210 (e.g., Si(f)) from the lips enters the mask (e.g. a SCBA mask), which has an acoustic transfer function 220 (e.g., MSK(f)) that is characterized by acoustic resonances and nulls. These resonances and nulls are due to the mask cavity volume and reflections of the sound from internal mask surfaces. These effects characterized by the transfer function MSK(f) distort the input speech waveform Si(f) and alter its spectral content. Another sound source is noise 230 generated from the breathing equipment (e.g. regulator inhalation noise) that also enters the mask and is affected by MSK(f). Another transfer function 240 (e.g., NPk(f)) accounts for the fact that the noise is generated from a slightly different location in the mask than that of the speech. The speech and noise S(f) are converted from acoustical energy to an electronic signal by a microphone which has its own transfer function 250 (e.g., MIC(f)). The microphone signal then typically passes through an audio amplifier and other circuitry, which also has a transfer function 260 (e.g., MAA(f)). An output signal 270 (e.g., So(f)) from MAA(f) may then be input into a radio for further processing and transmission.
Returning to the shortcomings of systems such as system 100, an example of such a shortcoming relates to the generation by these systems of loud acoustic noises as part of their operation. More specifically, these noises can significantly degrade the quality of communications, especially when used with electronic systems such as radios. One such noise that is a prominent audio artifact introduced by a pressurized air delivery system, like a SCBA system, is regulator inhalation noise, which is illustrated in FIG. 2 as box 230.
The regulator inhalation noise occurs as a broadband noise burst occurring every time the mask wearer inhales. Negative pressure in the mask causes the air regulator valve to open, allowing high-pressure air to enter the mask and producing a loud hissing sound. This noise is picked up by the mask communications system microphone along with ensuing speech, and has about the same energy as the speech. The inhalation noise generally does not mask the speech since it typically occurs only upon inhalation. However, it can cause problems—examples of which are described as follows. For example, the inhalation noise can trigger VOX (voice-operated switch) circuits, thereby opening and occupying radio channels and potentially interfering with other speakers on the same radio channel. Moreover, in communication systems that use digital radios, the inhalation noise can trigger VAD (Voice Activity Detector) algorithms causing noise estimate confusion in noise suppression algorithms farther down the radio signal processing chain. In addition, the inhalation noise is, in general, annoying to a listener.
A second shortcoming of systems such as system 100 is described below. These systems use masks that typically encompass the nose and mouth, or the entire face. The air system mask forms an enclosed air cavity of fixed geometry that exhibits a particular set of acoustic resonances and anti-resonances (nulls) that are a function of mask volume and internal reflective surface geometries, and that alters the spectral properties of speech produced within the mask. More specifically, in characterizing the air mask audio path (FIG. 2), the most challenging part of the system is the acoustic transfer function (220) from the speaker's lips to the mask microphone. These spectral distortions can significantly degrade the performance of attached speech communication systems, especially systems using parametric digital codecs that are not optimized to handle corrupted speech. Acoustic mask distortion has been shown to affect communication system quality and intelligibility, especially when parametric digital codecs are involved. Generally, aside from the inhalation noise, the air system effects causing the largest loss of speech quality appear to be due to the poor acoustics of the mask.
FIG. 3 illustrates an example of a measured spectral magnitude response inside the mask (320) and at the mask microphone output (310) and a calculated combined transfer function (330) for the mask, microphone, and microphone amplifier. These particular data were obtained using a SCBA mask mounted on a head and torso simulator. The acoustic excitation consisted of a 3 Hz-10 KHz swept sine wave driving an artificial mouth simulator. As FIG. 3 illustrates, the spectrum is significantly attenuated at frequencies below 500 Hz and above 4.0 KHz, mostly due to a preamp band pass filter in the microphone, and contains a number of strong spectral peaks and notches in the significant speech pass band region between 50 and 4.0 KHz. These spectral peaks and notches are generally caused by reflections inside the mask that cause comb filtering, and by cavity resonance conditions. The significant spectral peaking and notching modulate the speech pitch components and formants as they move back and forth through the pass band, resulting in degraded quality and distorted speech. It may be desirable to determine a transfer function or transfer functions characterizing such a system with such transfer functions being used to define an equalization system to reduce speech distortion.
A number of proven techniques exist to adaptively determine a system transfer function and equalize a transmission channel. One effective method to determine a system transfer function is to use a broadband reference signal to excite the system and determine the system parameters. A problem in estimating the transfer function of many speech transmission environments is that a suitable broadband excitation signal is not readily available. One common approach is to use the long-term average speech spectrum as a reference. However, adaptation time using this reference can take a long time, particularly if the speech input is sparse. In addition, the long-term speech spectrum can vary considerably for and among individuals in public service activities that frequently involve shouting and emotional stress that can alter the speech spectrum considerably.
Another shortcoming associated with systems such as system 100 is the lack of more efficient methods and apparatus for measuring certain parameters of the mask wearer including, for example, biometric parameters. Measurement of such parameters of individuals working in hazardous environments, who may be using systems such as system 100, is important for monitoring the safety and performance of those individuals. For example, measurements of the individual's respiration rate and air consumption are important parameters that characterize his work-load, physiological fitness, stress level, and consumption of the stored air supply (i.e. available working time). Conventional methods of measuring respiration involve the use of chest impedance plethysmography or airflow temperature measurements using a thermistor sensor. However, getting reliable measurements, using these conventional methods, from individuals working in physically demanding environments such as firefighting is more difficult due to intense physical movement that can cause displacement of body-mounted sensors and artifacts typically used to take the measurements.
Thus, there exists a need for methods and apparatus for effectively detecting and attenuating inhalation noise, equalizing speech (i.e., removing distortion effects), and measuring parameters associated with users in a system that includes a pressurized air delivery system coupled to a communication system.